Field
The present specification generally relates to petroleum processing and, more specifically, to integrated systems and methods for separating, extracting, and recovering polynuclear aromatic hydrocarbons, heterocyclic compounds, and organometallic compounds from a hydrocarbon feedstock.
Technical Background
Crude oil or petroleum refineries are integrations of complex processes in which crude oil and its fractions are processed by various unit operations and unit processes. A conventional refinery primarily produces transportation fuels such as liquefied petroleum gas (LPG), diesel, gasoline, aviation fuel, kerosene, and fuel oils, for example. Some refineries may also produce bitumen, asphaltenes, and aromatics. Still other petroleum refineries produce lube oils, anode grade coke, and BTX (benzene, toluene, xylene) products, depending on the type of crude they are processing. New generation refineries also produce olefins as petrochemical feedstock in addition to BTX products.
However, the major products that come from refineries are the transportation fuels. With newer and stricter legislations on SOx, NOx and particulate emissions the hydrotreatment requirement of the transportation feedstock is becoming more challenging for the refiners. Refineries consume a significant amount of energy (7-15%) during processing of crude oil and more specifically while hydrotreating the transportation fuels.
Refiners are processing heavier crude (lower API gravity) as the supply of light crude is dwindles. To improve the yield of transportation fuel, e.g., the light and mid-distillate, the refiners are cracking the higher boiling point resin fractions of heavy and medium crude. Moreover, the heavy distillates (cracked or vacuum) and deasphalted oil (DAO) contain very high quantities of heterocyclics and polynuclear aromatic hydrocarbons (PAHs) and as a result, significant quantities of heterocyclics and PAHs end up in the cracked heavy and mid distillates, whereas the low molecular weight aromatics and heterocyclics end up in light cracked distillates.
The increased presence of heterocyclic compounds and PAHs in the crude supply is placing increasingly stronger constraints on hydrotreatment of the cracked distillates coming from the heavy fractions. For example, Arab Heavy contains 2.78 wt. % sulfur in virgin crude, whereas Arab Medium contains 1.4 wt. % sulfur in its crude (on elemental S basis). On average, a minimum of 5-10 wt. % (as organic S compounds) of hydrocarbon is chemically bound with sulfur (S) and nitrogen (N) heterocyclics in heavy and medium crude oil in addition to PAHs, mostly in the resins and asphaltenes. Moreover, owing to higher boiling point of resins and asphaltenes, the heterocyclic and organometallic compounds and PAHs end up in the heavy fractions after the fractionation in the Atmospheric and Vacuum distillation column. Because the PAHs, organometallic, and heterocyclic compounds are chemically bound in the larger macromolecules such as resins and asphaltenes, the direct recovery or extraction of these larger macromolecules is not convenient or profitable and commercially attractive from the atmospheric or vacuum resin fractions. Macromolecules (from resins) must be further cracked so that only the smaller PAHs and heterocyclic and organometallic compounds with minimal side chains can be separated once again before commercially marketing them as feedstocks for fine chemicals as smaller fragmented molecules.
To obtain increased quantities of mid-distillate, the heavier fractions (atmospheric bottoms and vacuum residue) of crude are cracked depending on the refinery configuration and type of crude. As a result, the heterocyclics and PAHs from resins and, to some extent, from asphaltenes end up as smaller fragmented molecules after the cracking. A significant number of heterocyclic and PAH fractions (10 wt. % or more, as organic compound basis) remain in the mid-distillates, heavy distillates and, to some extent, the light distillate after this cracking. Therefore, concentrations of the organic heterocyclic compounds, organometallic compounds, and 2-4 cycle PAH compounds are significantly increased in mid and heavy distillate fractions. The light and mid distillates from the refining operation are sent for hydrotreatment (HDT) to remove the compounds to produce transportation fuels (gasoline and diesel) lower in sulfur, nitrogen, and metals. But during the conventional hydrotreatment (HDT) of crude fractions, e.g., especially mid-distillate (diesel pool) and light distillate (gasoline pool), the heterocyclic compounds are converted to hydrocarbon molecules, free from sulfur and nitrogen, whereas the PAHs are converted to aromatics and/or saturated cyclics.
Notwithstanding their troublesome nature when left in petroleum products such as transportation fuels, for example, the PAHs and organic heteroatom compounds possess exotic properties. In particular, they are optically active, electrically active, chemically active, and have interesting semiconducting properties and radio-frequency properties. They also have high value in several technical markets. The same compounds lose their exotic properties during conventional hydrotreatment processes in refineries due to the saturation of their conjugated bonds.
Polynuclear aromatic hydrocarbons, refractory heterocyclic organic compounds containing sulfur and/or nitrogen, and organometallic compounds are valuable chemical feedstocks for many applications. Such compounds find uses in production of fine chemicals or as building blocks for organic solar cells, organic LEDs, other organic thin-film transistors, ultra-high performance batteries, for example. Various derivatives of such compounds are also finding their places in research environments for industries such as consumer electronics and renewable energy. Though most or all of these compounds are found naturally in hydrocarbon feedstocks such as crude oil, crude fractions, and petroleum sources, for example, conventional methods of petroleum production or refining typically either cause the compounds to go to waste, to be left as minor impurities in other products without capitalizing on the additional value of the compounds in isolation, or to be removed from the petroleum source but chemically converted to sulfur, nitrogen and metal free organic hydrocarbon during the removal.
As the overall crude oil supply around the world is diminishing, the existing crude oil supply is becoming heavier all across the globe. The presence of PAHs, the refractory heterocyclic organic compounds, and the organometallic compounds is much greater in heavy crude oil than in lighter crude oil. In turn, conventional hydrodesulfurization/hydrodenitrogenation (HDS/HDN) processes or demetallization processes typically used to remove such compounds from crude petroleum are being strained, especially in terms of increased cost. The cost of HDS/HDN rises with respect to the amount of the compounds in the crude oil, because HDS/HDN or demetallization requires higher severity and higher hydrogen consumption to remove greater amounts of the compounds.
On the other hand, if PAHs, refractory heterocyclic organic compounds, and organometallic compounds can be removed from crude oil or crude fractions at lower severity and without destroying their molecular structures, two benefits can be realized. First, the compounds may be provided for further applications. Second, the total cost of HDN/HDS to eliminate nitrogen and sulfur from the crude fractions can be drastically reduced. Accordingly, ongoing needs exist for systems and methods that remove PAHs, refractory heterocyclic organic compounds containing sulfur and/or nitrogen, and organometallic compounds from crude oil or crude fractions without destroying the molecular structures thereof so that the compounds may be used or kept available for other applications.